Petrography of orbicular granitoids in the Separation Point

Abstract

Abstract Eleven occurrences of orbicular fabric in the (mostly) coastal exposures of the Separation Point Granite in the Motueka region, northwest Nelson, are described. Alkali-feldspar phenocrysts, biotite clusters, and orbicules may be concentrated locally. The orbicules, from 3 cm to over 20 cm in diameter, occur as a few to many individuals or as densely packed aggregates (orbiculite) in a granitic matrix similar to the main rock. Orbicules consist of a spheroidal or squarish core either alone or with one to several concentric shells. Cores are essentially plagioclase (often radiating mosaics), whereas shells are either coarse alkali-feldspar (with quartz) or concentrations of magnetite and biotite arranged concentrically or tangentially through the persisting plagioclase, and usually of increasing abundance towards the edges of the orbicules. The orbicules may be elliptically deformed, or even fragmented, indicating a dynamic environment of formation and accumulation in the magma during its consolida...